When an ejection seat is jettisoned from the cockpit of an aircraft, it must pass through the region occupied by the transparent canopy of the aircraft. In instances where the canopy is not jettisoned prior to the ejection seat, the ejection seat must be capable of blasting entirely through the canopy. To reduce the risks to the pilot or other aircraft occupant attendant to forcing the ejection seat through the canopy, canopy fracture systems have been provided to fracture the canopy and better clear a path for the ejecting occupant so as to minimize bodily impact with the canopy. The risks attendant to ejection from a moving aircraft are particularly pronounced when the pressure on the exterior of the aircraft canopy exceeds the pressure within the cockpit so that the canopy experiences a negative pressure environment. In such circumstances, the possibility exists that any severed portion of the canopy may implode through the opening created in the canopy and enter into the cockpit.
Canopy fracturing systems may utilize fragilizing or non-fragilizing canopies. For purposes of the present disclosure, "fragilizing" aircraft canopies are those that may be caused to shatter into a significant number of pieces on application of sufficient pressure or explosive force. Such canopies typically are constructed of cast materials including, for example cast acrylic. "Non-fragilizing" aircraft canopies are constructed of materials that do not shatter on application of explosive force but, rather, may be caused to fracture along a predetermined pattern, referred to herein as a "fracture pattern", such that at least a portion of the canopy may be severed and freed from the aircraft and at least a portion of the ejecting occupant may pass relatively unimpeded through the opening fractured in the canopy. Examples of non-fragilizing canopy materials are stretched acrylic and polycarbonate. The fracturing of a non-fragilizing canopy, such as, for example, a stretched acrylic canopy, using the canopy fracturing system of the present invention will not result in the shattering of the canopy or a portion thereof into a significant number of fragments. However, small shards of the non-fragilizing canopy material localized to the source of fracturing energy may be liberated during fracturing.
Several canopy fracturing systems utilizing various fracture patterns are known for facilitating the emergency egress of aircraft pilots and other cockpit occupants. A canopy fracturing system available from Saab Scania and generally described in Swedish patent application number 502678 is utilized in JAS 39B aircraft. The Saab Scania canopy fracturing system utilizes an air bag in the aft canopy of two-seat, dual canopy JAS 39B aircraft to protect the pilot from imploding sections of the canopy following the fracturing of the canopy by a linear shape charge along an S-shaped path during the egress event. Certain features of the Saab Scania canopy fracturing system are depicted in FIG. 1, in which aft canopy 100 includes a canopy frame 110, transparent portion 112, and a linear explosive charge 114 mounted to the transparent portion of the canopy in an S-shaped pattern. The explosive charge 114, when detonated, fractures the transparent portion of the canopy along the S-shaped pattern into "clamshell" sections. Absent inclusion of the airbag in the Saab Scania system, the possibility exists that the severed clamshell sections, although initially forced away from the cockpit by the charge's explosive impulse, may be caused to implode by the aerodynamic field surrounding the canopy. The air bag acts to both drive the severed sections away from the pilot and to physically shield the pilot from those sections as they disintegrate, and it also facilitates aftward passage of the severed sections by action of the airstream passing over the canopy.
Another known canopy fracturing system utilizes a D-shaped fracture pattern. Certain features of that known design is depicted in FIG. 2, in which the canopy is generally designated as 120 and includes transparent portion 124 having explosive charges mounted in D-shaped fracture pattern 126. The D-shaped fracture pattern design successfully clears a path for an ejecting occupant when the aerodynamic field surrounding the canopy is neutral or positive, such that the field assists in the flyaway of the severed D-shaped canopy section, but the system is not as effective in ensuring suitable flyaway of the severed section in negative pressure conditions.